Currently, a DVD (digital versatile disk), which is an optical recording medium having a large storage capacity for which data writing or re-wiring is allowed, exists. A storage capacity of a current DVD-R (recordable) or a DVD-RW (re-writeable) is 4.7 GB (gigabytes) for one side in a single layer, and it has a pattern dimension in which a groove width is on the order of 400 nm, and a track pitch (a width between grooves) is on the order of 740 nm.
In contrast thereto, as a next-generation DVD, an HD-DVD (high definition DVD) exists. A storage capacity of HD-DVD is on the order of 15 GB for one side in a single layer, has a groove width on the order of 200 through 300 nm, and a track pitch on the order of 600 nm. Further, a storage capacity of a Blu-ray Disk is on the order of 23 through 27 GB for one side in a single layer, has a groove width on the order of 140 through 170 nm, and a track pitch on the order of 320 nm.
Further, a further-next-generation DVD is expected as having a storage capacity on the order of 50 through 100 GB for one side in a single layer. Its groove width may be less than 100 nm, and its track pitch may be on the order of 200 through 300 nm.
For a current DVD, data writing is carried out with an optical mastering apparatus such as a LBR (laser beam recording apparatus). Also for a next-generation HD-DVD, it is expected that writing with an optical mastering apparatus can be made. However, since a pit pattern of a next-generation Blu-ray Disk or a pattern dimension of a further-next-generation DVD is very small, data writing cannot be carried out by an optical mastering apparatus.
Accordingly, in order to write data in a next-generation Blu-ray Disk or a further-next-generation DVD, an electron beam mastering apparatus such as an EBR (electron beam recording apparatus) or such generating an electron beam with a very small beam diameter with a large electric current is required. For example, an electron beam mastering apparatus generating an electron beam with a beam diameter not more than 70 nm, with a large electric current of not less than 400 nA is required.
With reference to FIG. 5, an electron beam applying apparatus in the related art is described.
In an electron beam applying apparatus shown in FIG. 5, an electron beam emitted from an electron source 52 undergoes correction for an axis shift thereof, passes through a hole part of a selector aperture (blanking aperture) 60, and after that, is condensed in a crossover point CP by a condenser lens 58. Then, the electron beam from the crossover point CP passes through a hole part of an objective aperture 61, undergoes correction for astigmatism thereof by means of an astigmatism correction coil 62, undergoes correction for a focus thereof by means of an objective lens 66, and is condensed on a surface of a material 70.
When information is written on the material 70, turning on/off of the electron beam emitted by the electron source 52 is controlled by blanking electrodes 54 and the selector aperture 60. Further, the electron beam having passed through the hole part of the selector aperture 60, the condenser lens 58 and the hole part of the objective aperture 61 and applied to the objective lens 66 is deflected by electrostatic deflection electrodes 64 according to information to be written, and thus, a position of a beam spot produced on the material 70 is controlled. That is, the surface of the material 70 is scanned by the electron beam, and thus, information is written in a predetermined position.
In the above-described electron beam applying apparatus 50 in the related art, two methods for increasing an amount of an electric current of the electron beam condensed on the surface of the material 70 can be considered, as follows:
As a first method, as shown in FIG. 6, an effective angular aperture of the electron beam emitted from the electron source 52 is increased, and also, an aperture diameter of the selector aperture 60 is increased (the passage of the electron beam in FIG. 6 is in a zone defined by the inner lines through a zone defined by the outer lines). By increasing the effective angular aperture of the electron beam emitted from the electron source 52, more part of the electron beam can be condensed by the condenser lens 58. Further, by increasing the aperture diameter of the selector aperture 60, more part of the electron beam can be condensed by the objective lens 66. Thereby, an amount of electric current of the electron beam condensed on the surface of the material 70 can be increased.
However, in this method, the angular aperture of the objective lens 66 increases as the aperture diameter of the selector aperture 60 is increased. As spherical aberration of a lens increases in proportion to third power of its angular aperture, a shift in the focal point increases due to a difference in the electron beam passage condensed on the surface of the material 70, occurring due to the spherical aberration of the objective lens 66. Accordingly, it becomes difficult to sufficiently condense the electron beam by means of the objective lens 66, and thus, it may not be possible to sufficiently reduce the beam diameter, or the beam diameter of the electron beam increases.
In the second method, as shown in FIG. 7, the angular aperture of the objective lens 66 is not changed, but the crossover point CP is moved downward (for example, from CP1 to CP2), and thus, a reduction ratio of the beam diameter of the electron beam condensed on the surface of the material 70 with respect to the beam diameter of the electron beam emitted from the electron source 52 is lowered (the passage of the electron beam is, in FIG. 7, in a zone defined by the inner lines through a zone defined by the outer lines). By lowering the beam diameter reduction ratio of the electron beam, the effective angular aperture of the electron beam emitted from the electron source 52 increases, and thus, it is possible to increase the electric current amount of the electron beam condensed on the surface of the material 70.
However, since the beam diameter reduction ratio of the electron beam is thus made smaller, this method can be applied only for a case where the beam diameter of the electron beam emitted by the electron source 52 is originally small, and this method cannot be applied for a case where the beam diameter is originally large, and thus, the beam diameter should be reduced. Further, as the beam diameter reduction ratio of the electron beam is thus made smaller, a positional shift of the electron source 52 due to a possible vibration, a positional shift of the electron source 52 due to a leading voltage (slight voltage shift) or such cannot be ignored, and such a positional shift of the electron source 52 may cause somewhat variation in the electron beam condensed on the surface of the material 70.
Further, as shown in FIG. 5, in the electron beam applying apparatus in the related art, generally, the two apertures, i.e., the selector aperture 60 and the objective aperture 66 are applied. The selector aperture 60 defines the angular aperture of the electron beam applied to the condenser lens 58 from the electron source 52, while the objective aperture 61 defines the angular aperture of the electron beam applied to the objective lens 66 from the crossover point CP′. That is, the selector aperture 60 and the objective aperture 61 substantially reduce the electric current amount of the electron beam condensed on the surface of the material 70.
Further, as the two apertures are thus applied, a mechanical error may occur therebetween, and the hole parts thereof may not be aligned on the same axis accurately. Therefore, the hole diameter of the hole part of the selector aperture 60 should be enlarged to include this mechanical error. However, if the hole diameter of the hole part of the selector aperture 60 is thus enlarged, the deflection amount of the electron beam for blanking increases. As a result, a relatively high voltage is required as a blanking voltage, and thus, an increase of a turning on/off switching frequency of the electron beam for blanking becomes difficult. That is, an increase in the drawing speed may not be possible.
Further, in the electron beam applying apparatus 50 in the related art, as shown in FIG. 5, the blanking electrodes 54 and the axis aligning coils 56 are disposed in close proximity. Conventionally, since a large electric current amount of the electron beam is not required, the electric current amount flowing through the blanking electrodes is small. However, when the electric current amount of the electron beam is increased, a large electric current should be made to flow through the blanking electrodes 54, for the purpose of increasing the turning on/off switching frequency of the electron beam for blanking.
However, when a large electric current is flown through the blanking electrodes 54, a magnetic field occurs in the axis aligning coils 56 disposed in close proximity thereto. Thereby, the electron beam is bent, and thus, an axis shift may occur in the electron beam.
Further, conventionally, since a large electric current is not required for the electron beam, the crossover point CP is set at a relatively higher position. Therefore, the beam diameter reduction ratio of the electron beam can be set in a relatively large amount. Thereby, even when somewhat axis shift occurs in the axis aligning coils 56, an influence of the axis shift of the electron beam condensed on the surface of the material 70 can be reduced by several times according to the large reduction ratio. Thus, an influence of the electron beam axis shift in the axis alignment coils 56 does not cause an actual problem.
However, when the crossover point CP is lowered for example as mentioned above for the purpose of increasing the electric current amount of the electron beam, the beam diameter reduction ratio of the electron beam lowers. Thereby, an axis shift of the electron beam in the axis aligning coils 56 may cause a large problem in this case.
As a prior art concerning the present invention, Japanese Laid-open-Patent Application No. 6-131706 is cited. This document discloses an information recording apparatus producing an original disk of an information recording medium such as an optical disk. For example, in FIG. 1 thereof, a configuration is disclosed in which an electron emitted by a filament is made to pass through Welnelt electrode and anode as an electron beam applying tube, after that the electron beam is condensed by first and second electromagnetic lenses, further, deflection electrodes are applied, and thus, the electron beam is focused on the original disk serving as a target.